1. Field of the Invention
The invention relates to processing circuitry for discrete-sample-type-color-video signals and more particularly to such circuitry which reduces artifacts of sampling and improves, at least apparently, the image detail of images resulting from such video signals.
2. Description Relative to the Prior Art
The conversion of a color image to an electrical signal representation by a color video imager typically involves either an elemental spot that scans continuously along a raster pattern or a grid of elemental areas that detect image information. In either case, the finite character of the sensing aperture results in a loss of image information. Because the image information that is successfully extracted by an imaging device is typically rather limited, specialized signal processing techniques are employed to maintain and to enhance, insofar as is possible, the quality of image representation.
For example, when image-representative signals are generated using a line scan process, as in a vidicon, it is known that certain degradation occurs as a result of the finite diameter of the scanning beam. Considering as an example a vertical edge in an image, only the leading portion of the beam initially "sees" the edge. Eventually the entire beam sweeps past the edge and is influenced by the edge transition. As a consequence of such operation, an abrupt edge transition tends to have a gradual influence on the scanning beam and, in effect, the edge is blurred. A somewhat similar effect, it will be appreciated, results with any image detail in the size range of the scanning beam.
To counteract this blurring effect, various "aperture correction circuits" (see e.g. U.S. Pat. No. 3,789,133) have been developed which function to amplify selectively high frequency scene information. By such boosting of high frequencies, these correction circuits reduce the undesirable loss of sharpness, albiet with some risk of amplifying noise inherent in the scanning device. It is also known in image signal processing that the green signal is closely related to the luminance response of the human visual system and may be aperture corrected, and used as the sole source of information regarding high-spatial-frequency image detail (see U.S. Pat. No. 3,732,360).
While such aperture correction techniques have been effective in upgrading the image-representative signals in line-scan-type systems such as single- and multi-tube color vidicons, they encounter problems when applied to the signals of elemental-area-type imagers, such as charge-transfer imaging devices, where image information is extracted as discrete samples. Such imagers provide discrete "updates," typically along respective element rows, rather than provide continuous line scan signals. The contrast in signal characteristics is even more significant where color imaging is performed on a "single chip" (see U.S. Pat. No. 3,971,065 and "Integral Color Filter Arrays for Solid State Imagers," Dillon et al, IEEE Int'l. Elec. Dev. Mtg. Conference Papers, Washington, D.C., 1976), because signal updates are staggered among the colors, and even the sampling frequency may vary with color.
As a result of the discrete sampling of such solid-state imagers, the color signals tend to exhibit sharp transitions at the boundaries of picture elements and mere amplification of high frequencies would only serve to worsen the artifacts of discrete sampling. Hence, known aperture correction techniques are not directly applicable to discrete area imaging devices and only tend to degrade the signals from such devices.